Field of the Invention
The present invention relates to an impedance matching circuit that is suitably connected to an antenna of portable wireless devices and the like, for example, that require impedance matching, and to an antenna system.
Description of Related Art
In electronic devices that conduct wireless communication such as mobile phones, as disclosed in Patent Documents 1, 2, and 3, for example, an impedance matching circuit is disposed between an antenna and a transmission/reception circuit. By providing the impedance matching circuit, the impedance of the antenna side is set so as to match with the impedance of the transmission/reception circuit side, and typically, the impedance is set to approximately 50Ω.
Patent Document 1 discloses an impedance matching circuit that is generally used in wireless devices and the like. FIG. 3 of Patent Document 1 shows an example of a Π-type impedance matching circuit in which an inductor is connected in series between an antenna and a transmission/reception circuit, and capacitors are connected in parallel to the front and the rear of the inductor. The capacitors are also connected to a ground.
When mobile phones are in use, an antenna thereof is placed near human body or other mobile devices, and therefore, unlike when the mobile phone is placed in a free space, the antenna impedance changes, which causes impedance mismatch with the transmission/reception circuit, and as a result, the antenna characteristics change. The change in antenna characteristics means a reduction in antenna efficiency, and is caused by a change in reflection coefficient or gain deterioration. A change in reflection coefficient causes not only the antenna characteristics to change, but also the passband loss in the transmission/reception bands to increase significantly. Therefore, the mismatch caused by a surrounding environment such as human body needs to be compensated by an impedance matching circuit, to suppress the deterioration of the antenna characteristics.
However, the mismatch of the antenna impedance that has been changed by the surrounding environment such as human body is too large to be addressed by a circuit that combines elements with fixed values. Therefore, in order to flexibly address the impedance change and achieve impedance matching, a method is employed in which a capacitance of each element included in the impedance matching circuit is made variable. For the frequencies that are commercially used for mobile phones, there is no usable variable inductor, and therefore, as disclosed in FIG. 1 of Patent Document 1, variable capacitors such as varactor diodes are used.
It is known that the degree of impedance mismatch can be quantified by VSWR (voltage standing wave ratio). Specifically, when VSWR is 1, the reflection coefficient becomes 0, which represents perfect matching, and as the degree of mismatch increases, VSWR becomes greater than 1. If the frequency at which VSWR is at a minimum is shifted due to a surrounding environment such as human body, then VSWR deteriorates. Thus, according to the impedance matching circuit shown in FIG. 1 of Patent Document 1, values of elements that constitute the circuit can be adjusted such that VSWR is at a minimum in a desired frequency.
Currently, a usable variable inductor that is small and has a low loss (high Q), which can be provided in mobile wireless devices, is not yet available, and therefore, a study is conducted on an impedance matching circuit that uses variable capacitors instead. FIG. 1 of Patent Document 2 discloses an impedance matching circuit that includes three variable capacitors. With the technique disclosed in Patent Document 2, if the variable ratio of the variable capacitance can be made 20 to 200 or greater, then it is possible to realize a circuit that is capable of impedance matching regardless of a region on the Smith chart to which the antenna impedance has been moved as a result of the impedance change. Being able to cover a large region on the Smith chart means that the circuit is not affected by a shape and characteristics of an antenna, or in other words, it is possible to realize a versatile impedance matching circuit that can be used for any antenna.
However, the variable ratio that can be achieved by variable capacitors that are currently available such as variable capacitance diodes, RF-CMOS, and ferroelectric varactor diodes is 10 or less, and when the variable ratio is 10 or less, the region that can be covered on the Smith chart is made smaller. Furthermore, if the control voltage for the variable capacitors such as variable capacitance diodes and ferroelectric varactor diodes is reduced, the variable ratio is further reduced to 3 or less, for example. Thus, Patent Document 3 proposes a circuit that can change the reactance and susceptance indefinitely by using variable capacitors that have definite variable range, for example.